While questioning the citizens, you overhear a call from Captain Aaron Sommers informing them that he is in position at C-Sec and needs them to distract the security while he moves in to make the kill.

Go into the nearby C-Sec office and speak to Captain Sommers. The Captain concludes that his plot has been exposed and explains that a C-Sec officer is in custody for aiding Cerberus during the coup attempt. The officer is giving intel on Cerberus to C-Sec in exchange for a lighter sentence, but Sommers argues that he deserves to die for having murdered innocent people. Since the officer is an Alliance veteran, like Sommers, and his actions dishonored the uniform, Sommers feels that it's up to him to deliver justice.

You can then choose how to resolve the situation:

Persuade Sommers to call off the hit (Paragon) - Shepard admonishes Sommers and points out that he would be dishonoring the Alliance uniform as well if he takes matters into his own hands. Sommers apologizes and shuts down the operation.

Sommers sometimes disappears while you're talking to him, making it look like you're speaking to the couch. The cause for this is unknown and it is an unexpected occurrence. Reloading an earlier save may fix the issue.